Patent Documents 1 to 3 discloses spark plugs, in each of which at least one of a center electrode and a ground electrode has an electrode body containing a base metal and a noble metal tip containing a noble metal and welded to the electrode body.
In Patent Document 1, the ground electrode is produced by resistance welding the noble metal tip to the electrode body to form a radially outwardly flanged portion at a bottom of the noble metal tip, and then, laser welding the noble metal tip to the electrode body with the irradiation of a laser onto the flanged portion of the noble metal tip.
In Patent Document 2, the ground electrode is produced in the following manner. The noble metal tip and the electrode body are irradiated with a pulsed laser to thereby weld the noble metal tip and the electrode body via a circumferential laser welding area in which welding spots corresponding to the respective pulses of the laser sequentially overlap each other in a circumferential direction of the noble metal tip. At this time, the laser irradiation energy per pulse is set lower for the formation of at least either the front part or end part of the welding area than for the formation of the other part of the welding area.
In Patent Document 3, the ground electrode is produced by irradiating a laser etc. at a given incident angle onto the border of the noble metal tip and the electrode body and thereby welding the noble metal tip and the electrode body to each other.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-158323
Patent Document 2: Japanese Laid-Open Patent Publication No. 2002-231417
Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-135456
In Patent Documents 1 to 3, the welding of the noble metal tip and the electrode body is performed by the irradiation of the pulsed laser in which each pulse has a rectangular waveform with a uniform laser intensity Sc throughout a pulse width Tc from a start time Tsc to a end time Tec. The laser welding with high irradiation energy per pulse is particularly effective at welding structural components of different melting points e.g. welding the noble metal tip containing the noble metal and the electrode body containing the base metal. However, molten metal spatters may occur under the laser irradiation and fly and adhere to the noble metal tip and the electrode body. This results in problems such as a deterioration in ignition performance of the spark plug and formation of a bridge in a discharge gap of the spark plug. Blowholes may also occur under the laser irradiation. In the case of forming the circumferential welding area in which the welding spots corresponding to the respective laser pulses overlap and continue into each other by the irradiation of the pulsed laser, it is particularly likely that the spatters and blowholes will occur during the formation of the latter welding sports. In addition, the metals molten by the laser irradiation get suddenly cooled and solidified so that cracks are likely to occur in the welding area during the solidification. This leads to a problem of separation of the noble metal tip from the electrode body due to a deterioration in joint strength of the welding area.
In this way, the conventional laser welding of the noble metal tip and the electrode body has the problems of the occurrence of the spatters, blowholes, cracks etc. As solutions to these problems, it is conceivable to adjust the welding state of the noble metal tip and the electrode body by changing the laser irradiation conditions such as the pulse width Tc and laser intensity Sc appropriately depending on the materials, shapes and dimensions of the noble metal tip and the electrode body. However, the above defect occurrence problems cannot be prevented sufficiently only by the changing of the laser irradiation conditions.